1. Field of the Invention
The present invention relates to a stop valve employed in a bleeding system of a fuel tank for an internal combustion engine of a motor vehicle and, more particularly, to an improvement in a stop valve having a float.
2. Description of the Related Art
Most of the fuel tanks of internal combustion engine vehicles are provided with a bleeding system that communicates the tanks with the atmosphere. Such a bleeding system is normally provided with a canister for preventing fuel vapor from escaping to the atmosphere.
If the bleeding system is exposed to the outside when the vehicle (or the fuel tank) tilts beyond a certain degree or flips over, fuel from the tank will likely flow through the system to the outside. Therefore, the bleeding system is provided with a stop valve for closing the system if the vehicle is significantly tilted or flipped over.
A typical stop valve employed in the fuel vapor bleeding system has a float that floats in liquid fuel to close the system when the fuel surface rises to a predetermined level.
A known stop valve (Japanese Laid-Open Patent Application No. HEI 2-112658) will be described with reference to FIGS. 12 and 13. FIG. 12 is a sectional view of such a stop valve, and FIG. 13 is an exploded perspective view of a float unit shown in FIG. 12.
The stop valve comprises a casing 1 and a float 2 provided therein. A spring 4 is disposed between the float 2 and a barrier plate 3 so as to urge the float 2 toward a valve opening 5. The casing 1 defines a bleeding port 6 with the valve opening 5 that serves as an inlet to the bleeding port 6. A valve element 7 for closing the valve opening 5 is coupled to a mounting member 8 that is connected to the float 2. The valve element 7 defines a hole 9 extending therethrough, which will be closed by the float 2.
The stop valve operates as follows. The bleeding port 6 is in communication with the atmosphere via a canister (not shown). The canister captures or absorbs fuel vapor from the bleeding port 6, thus preventing the fuel vapor from escaping to the atmosphere.
If the fuel tank is tilted or inclined beyond a certain degree and fuel enters the casing 1, the float 2 rises so that the valve element 7 closes the valve opening 5 and, simultaneously, the float 2 closes the hole 9 of the valve element 7. Thus, fuel will not leak to the outside via the bleeding system.
When the fuel tank resumes a normal posture and the fuel flows out from the casing 1 toward the tank, the float 2 will descend by gravity, separating from the hole portion 9 of the valve element 7. Even if the tank pressure has become higher than the atmospheric pressure by that time, the attachment of the float 2 to the hole portion 9 will be readily released because the area of attachment between the float 2 and the hole portion 9 is small relative to the weight of the float 2. As a result, the pressure difference across the valve element 7 is canceled so that the valve element 7 is released from the valve opening 5.
Recent demand for increased bleeding capacity has increased the size of the valve opening 5. Accordingly, the valve element 7 is also increased in diameter. Such increased dimensions or area increases the force that will act on the valve element 7 toward the valve opening portion 5 when the tank pressure becomes higher than the atmospheric pressure with the valve opening 5 closed by the valve element 7. The increased closing force will impede separation of the valve element 7 from the valve opening portion 5. Therefore, the enlarged valve element 7 may remain stuck to the valve opening portion 5 even after the vehicle resumes a horizontal posture.
To avoid this undesired event, the stop valve shown in FIGS. 12 and 13 employs a two-step shutting construction as described above. In this construction, the float 2 closes the hole 9 formed in a central portion of the valve element 7, and a peripheral portion of the valve element 7 abuts a periphery of the valve opening 5. Thus, the valve opening 5 is closed by the valve element 7, and the hole 9 of the valve element 7 is closed by the float 2.
Even if the tank pressure is significantly higher than the atmospheric pressure, the float 2 will be readily separated from the hole portion 9 by gravity. Then, vapor flows through the hole 9 into the bleeding port 6 to cancel the pressure difference across the valve element 7. Thus, the valve element 7 will readily separate from the valve opening portion 5.
The valve element 7 is normally formed of an elastically deformable material, such as rubber, to enhance sealing of the bleeding system. However, when such an elastically deformable valve element 7 deforms, a central portion surrounding the hole 9 may also deflect, resulting in insufficient closure of the hole 9 by the float 2.
In the construction shown in FIGS. 12 and 13, the valve element 7 is mounted on the mounting member 8, which is connected to the float 2. The valve element 7 is thus supported so as to reduce deflection of the central portion surrounding the hole 9.
However, the above-described known stop valve has the following drawbacks.
Mounting the valve element 7 to the mounting member 8 does not sufficiently prevent deformation of the valve element 7. Thus, a central portion of the valve element 7 is likely to deform and impede sealing of the hole 9.
Employment of the mounting member 8 for reducing deformation of the valve element 7 adds to the number of component parts, thus increasing production costs and the number of assembly steps required.
Furthermore, if the diameter of the valve opening 5 is increased, splashing fuel produced during refueling or the like will more likely reach the canister via the bleeding port 6. To prevent such an event, the length of the bleeding system must be increased. Such a dimensional increase will reduce the freedom of designing or laying out the fuel tank system.